1. Field of the Invention
The present invention relates to a boundary acoustic wave device used, for example, as a band-pass filter or a resonator, and more particularly, to a boundary acoustic wave device having a three-medium structure in which first and second dielectric layers are laminated on a piezoelectric substrate.
2. Description of the Related Art
Since a package can be omitted and size reduction can be achieved, boundary acoustic wave devices have been attracting attention as alternatives to surface acoustic wave devices. International Publication No. 2007/85237 described below discloses a boundary acoustic wave device that utilizes an SH type boundary acoustic wave or a Stoneley wave as a boundary acoustic wave.
FIG. 8 is a schematic partially cutaway front cross-sectional view of the boundary acoustic wave device described in International Publication No. 2007/85237.
A boundary acoustic wave device 1001 includes a piezoelectric substrate 1002 and first and second dielectric layers 1003 and 1004 laminated on the piezoelectric substrate 1002. In other words, the boundary acoustic wave device 1001 is a boundary acoustic wave device having a three-medium structure.
An IDT electrode 1005 is located between the piezoelectric substrate 1002 and the first dielectric layer 1003. Here, the first dielectric layer 1003 is made of SiO2. The thickness of the first dielectric layer 1003 made of SiO2 is preferably in the range from 20% to 200% of the wavelength of the boundary acoustic wave, where the wavelength of the boundary acoustic wave is indicated by λ. It is stated that by setting the thickness of the SiO2 layer in this range, a spurious response can be suppressed and good filter characteristics can be obtained.
In a boundary acoustic wave device having a three-medium structure as described above, for example, when a Stoneley wave is utilized, a response in a higher-order mode of the Stoneley wave is good. In other words, a higher-order mode spurious response tends to appear. The magnitude of the higher-order mode spurious response can be reduced by reducing the thickness of the SiO2 layer.
However, while the thickness range of the SiO2 layer in which the higher-order mode spurious response is suppressed is different between the case in which the SH type boundary wave is utilized and the case in which the Stoneley wave is utilized, it is merely stated in International Publication No. 2007/85237 that when either boundary acoustic wave is utilized, the thickness of the SiO2 layer is preferably in the range from 20% to 200% of the wavelength λ.